PostgreSQL  JOIN 联表查询实战演练(内连接 / 外连接 / 交叉连接)

SELECT columns
FROM table1
JOIN table2 ON table1.column = table2.column
WHERE conditions;

-- 创建部门表
CREATE TABLE departments (
    dept_id SERIAL PRIMARY KEY,
    dept_name VARCHAR(100) NOT NULL
);
-- 创建员工表
CREATE TABLE employees (
    emp_id SERIAL PRIMARY KEY,
    emp_name VARCHAR(100) NOT NULL,
    dept_id INT, -- 外键关联到 departments 表
    salary DECIMAL(10, 2),
    FOREIGN KEY (dept_id) REFERENCES departments(dept_id)
);
-- 插入部门数据
INSERT INTO departments (dept_name) VALUES
('Human Resources'),
('Engineering'),
('Marketing'),
('Finance');
-- 插入员工数据
INSERT INTO employees (emp_name, dept_id, salary) VALUES
('Alice Johnson', 1, 75000.00),
('Bob Smith', 2, 85000.00),
('Carol Davis', 2, 90000.00),
('David Wilson', 3, 65000.00),
('Eve Brown', 1, 70000.00),
('Frank Miller', 4, 80000.00),
('Grace Lee', NULL, 55000.00); -- Grace 没有分配部门

SELECT e.emp_name, e.salary, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
ORDER BY e.emp_name;

SELECT e.emp_name, e.salary, d.dept_name
FROM employees e
LEFT JOIN departments d ON e.dept_id = d.dept_id
ORDER BY e.emp_name;

-- 创建项目表 (假设项目属于部门)
CREATE TABLE projects (
    project_id SERIAL PRIMARY KEY,
    project_name VARCHAR(100) NOT NULL,
    dept_id INT, -- 外键
    budget DECIMAL(12, 2)
);
-- 插入项目数据
INSERT INTO projects (project_name, dept_id, budget) VALUES
('Website Redesign', 1, 50000.00),
('Mobile App', 2, 100000.00),
('Market Research', 3, 25000.00),
('New Office Setup', 5, 75000.00); -- 部门ID 5 在 departments 表中不存在
-- 查询项目及其所属部门 (使用 RIGHT JOIN)
SELECT p.project_name, p.budget, d.dept_name
FROM projects p
RIGHT JOIN departments d ON p.dept_id = d.dept_id
ORDER BY d.dept_name;

SELECT e.emp_name, e.salary, d.dept_name
FROM employees e
FULL OUTER JOIN departments d ON e.dept_id = d.dept_id
ORDER BY e.emp_name, d.dept_name;

SELECT e.emp_name, d.dept_name
FROM employees e
CROSS JOIN departments d
ORDER BY e.emp_name, d.dept_name;

<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>42.6.0</version> <!-- 请检查最新版本 -->
</dependency>

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class DatabaseConnection {
    private static final String URL = "jdbc:postgresql://localhost:5432/your_database_name"; // 替换为你的数据库名
    private static final String USER = "your_username"; // 替换为你的用户名
    private static final String PASSWORD = "your_password"; // 替换为你的密码
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(URL, USER, PASSWORD);
    }
}

import java.sql.*;
import java.util.ArrayList;
import java.util.List;

// 用于存储查询结果的简单类
class EmployeeWithDept {
    private String employeeName;
    private Double salary;
    private String departmentName;

    public EmployeeWithDept(String employeeName, Double salary, String departmentName) {
        this.employeeName = employeeName;
        this.salary = salary;
        this.departmentName = departmentName;
    }

    // Getters and Setters
    public String getEmployeeName() { return employeeName; }
    public void setEmployeeName(String employeeName) { this.employeeName = employeeName; }
    public Double getSalary() { return salary; }
    public void setSalary(Double salary) { this.salary = salary; }
    public String getDepartmentName() { return departmentName; }
    public void setDepartmentName(String departmentName) { this.departmentName = departmentName; }

    @Override
    public String toString() {
        return "EmployeeWithDept{" +
                "employeeName='" + employeeName + '\'' +
                ", salary=" + salary +
                ", departmentName='" + departmentName + '\'' +
                '}';
    }
}

public class EmployeeReportService {

    public List<EmployeeWithDept> getEmployeesWithDepartments() throws SQLException {
        List<EmployeeWithDept> results = new ArrayList<>();
        String sql = """
            SELECT e.emp_name, e.salary, d.dept_name
            FROM employees e
            INNER JOIN departments d ON e.dept_id = d.dept_id
            ORDER BY e.emp_name
            """;

        try (Connection conn = DatabaseConnection.getConnection();
             PreparedStatement pstmt = conn.prepareStatement(sql);
             ResultSet rs = pstmt.executeQuery()) {

            while (rs.next()) {
                String empName = rs.getString("emp_name");
                Double salary = rs.getDouble("salary");
                String deptName = rs.getString("dept_name");
                results.add(new EmployeeWithDept(empName, salary, deptName));
            }
        }
        return results;
    }

    public static void main(String[] args) {
        EmployeeReportService service = new EmployeeReportService();
        try {
            List<EmployeeWithDept> employees = service.getEmployeesWithDepartments();
            System.out.println("=== 员工及其部门 (INNER JOIN) ===");
            for (EmployeeWithDept emp : employees) {
                System.out.println(emp);
            }
        } catch (SQLException e) {
            e.printStackTrace(); // 在实际应用中，应该使用更健壮的日志记录
        }
    }
}

public class EmployeeReportService {
    // ... (前面的方法)
    public List<EmployeeWithDept> getAllEmployeesWithDepartments() throws SQLException {
        List<EmployeeWithDept> results = new ArrayList<>();
        String sql = """
            SELECT e.emp_name, e.salary, d.dept_name
            FROM employees e
            LEFT JOIN departments d ON e.dept_id = d.dept_id
            ORDER BY e.emp_name
            """;
        try (Connection conn = DatabaseConnection.getConnection();
             PreparedStatement pstmt = conn.prepareStatement(sql);
             ResultSet rs = pstmt.executeQuery()) {
            while (rs.next()) {
                String empName = rs.getString("emp_name");
                Double salary = rs.getDouble("salary");
                String deptName = rs.getString("dept_name");
                results.add(new EmployeeWithDept(empName, salary, deptName));
            }
        }
        return results;
    }
    public static void main(String[] args) {
        EmployeeReportService service = new EmployeeReportService();
        try {
            List<EmployeeWithDept> employees = service.getAllEmployeesWithDepartments();
            System.out.println("=== 所有员工及其部门 (LEFT JOIN) ===");
            for (EmployeeWithDept emp : employees) {
                System.out.println(emp);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

=== 所有员工及其部门 (LEFT JOIN) ===
EmployeeWithDept{employeeName='Alice Johnson', salary=75000.0, departmentName='Human Resources'}
EmployeeWithDept{employeeName='Bob Smith', salary=85000.0, departmentName='Engineering'}
EmployeeWithDept{employeeName='Carol Davis', salary=90000.0, departmentName='Engineering'}
EmployeeWithDept{employeeName='David Wilson', salary=65000.0, departmentName='Marketing'}
EmployeeWithDept{employeeName='Eve Brown', salary=70000.0, departmentName='Human Resources'}
EmployeeWithDept{employeeName='Frank Miller', salary=80000.0, departmentName='Finance'}
EmployeeWithDept{employeeName='Grace Lee', salary=55000.0, departmentName='null'}

public class EmployeeReportService {
    // ... (前面的方法)
    public List<EmployeeWithDept> getEmployeesByDepartmentId(int deptId) throws SQLException {
        List<EmployeeWithDept> results = new ArrayList<>();
        String sql = """
            SELECT e.emp_name, e.salary, d.dept_name
            FROM employees e
            INNER JOIN departments d ON e.dept_id = d.dept_id
            WHERE d.dept_id = ?
            ORDER BY e.emp_name
            """;
        try (Connection conn = DatabaseConnection.getConnection();
             PreparedStatement pstmt = conn.prepareStatement(sql)) {
            pstmt.setInt(1, deptId); // 设置参数 ? 为传入的部门 ID
            try (ResultSet rs = pstmt.executeQuery()) {
                while (rs.next()) {
                    String empName = rs.getString("emp_name");
                    Double salary = rs.getDouble("salary");
                    String deptName = rs.getString("dept_name");
                    results.add(new EmployeeWithDept(empName, salary, deptName));
                }
            }
        }
        return results;
    }
    public static void main(String[] args) {
        EmployeeReportService service = new EmployeeReportService();
        try {
            // 查询 Engineering 部门的员工 (假设 dept_id = 2)
            List<EmployeeWithDept> engineeringEmployees = service.getEmployeesByDepartmentId(2);
            System.out.println("=== Engineering 部门员工 ===");
            for (EmployeeWithDept emp : engineeringEmployees) {
                System.out.println(emp);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

SELECT e.emp_name, d.dept_name, e.salary
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
WHERE e.salary > 75000;

SELECT e.emp_name, d.dept_name, p.project_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
INNER JOIN project_assignments pa ON e.emp_id = pa.emp_id
INNER JOIN projects p ON pa.project_id = p.project_id;

SELECT e.emp_name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id;

-- ❌ 错误示例
SELECT e.emp_name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
WHERE dept_id = 1; -- 错误！应为 e.dept_id 或 d.dept_id
-- ✅ 正确示例
SELECT e.emp_name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id
WHERE e.dept_id = 1;